OP181/OP281/OP481

–13–

REV. 0

Amplifier A1 is used as a voltage follower that will only track the

input voltage when it is greater than 0 V. This provides a half-

wave rectification of the input signal to the noninverting

terminal of amplifier A2. When A1’s output is following the

input, the inverting terminal of A2 will also follow the input

from the virtual ground between the inverting and noninverting

terminals of A2. With no potential difference across R1, no

current flows through either R1 or R2, therefore the output of

A2 will also follow the input. Now, when the input voltage goes

below 0 V, the noninverting terminal of A2 becomes 0 V. This

makes A2 work as an inverting amplifier with a gain of 1 and

provides a full-wave rectified version of the input signal. A 2 k

Ω

resistor in series with A1’s noninverting input protects the

device when the input signal becomes less than ground.

A Battery Powered Telephone Headset Amplifier

Figure 43 shows how the OP281 can be used as a two-way

amplifier in a telephone headset. One side of the OP281 can be

used as an amplifier for the microphone, while the other side

can be used to drive the speaker. A typical telephone headset

uses a 600

Ω speaker and an electret microphone that requires a

supply voltage and a biasing resistor.

+3V

OP281-B

20k

Ω

20k

Ω

Q1

Q2

+3V

1µF

600

Ω

SPEAKER

50k

Ω

10k

Ω

1µF

10k

Ω

POT.

+3V

1M

Ω

1M

Ω

1µF

1µF

INPUT

1M

Ω

+3V

OP281-A

11k

Ω

300k

Ω

0.1µF

1µF

+3V

1M

Ω

1µF

MIC OUT

2.2k

Ω

+3V

ELECTRET

MIC

Figure 43. A Battery Powered Telephone Headset

Two-Way Amplifier

The OP281-A op amp provides about 29 dB of gain for audio

signals coming from the microphone. The gain is set by the

300 k

Ω and 11 kΩ resistors. The gain bandwidth product of the

amplifier is 95 kHz, which, for the set gain of 28, yields a –3 dB

rolloff at 3.4 kHz. This is acceptable since telephone audio is

band limited for 300 kHz to 3 kHz signals. If higher gain is

required for the microphone, an additional gain stage should be

used, as adding any more gain to the OP281 would limit the

audio bandwidth. A 2.2 k

Ω resistor is used to bias the electret

microphone. This resistor value may vary depending on the

specifications on the microphone being used. The output of the

microphone is ac coupled to the noninverting terminal of the op

amp. Two 1 M

Ω resistors are used to provide the dc offset for

single supply use.

The OP281-B amplifier can provide up to 15 dB of gain for the

headset speaker. Incoming audio signals are ac coupled to a

10 k

Ω potentiometer that is used to adjust the volume. Again,

two 1 M

Ω resistors provide the dc offset with a 1 µF capacitor

establishing an ac ground for the volume control potentiometer.

Because the OP281 is a rail-to-rail output amplifier, it would

have difficulty driving a 600

Ω speaker directly. Here, a class AB

buffer is used to isolate the load from the amplifier and also

provide the necessary current drive to the speaker. By placing

the buffer in the feedback loop of the op amp, crossover

distortion can be minimized. Q1 and Q2 should have minimum

betas of 100. The 600

Ω speaker is ac coupled to the emitters to

prevent any quiescent current from flowing in the speaker. The

1

µF coupling capacitor makes an equivalent high pass filter

cutoff at 265 Hz with a 600

Ω load attached. Again, this does

not pose a problem, as it is outside the frequency range for

telephone audio signals.

The circuit in Figure 43 draws around 250

µA of current. The

class AB buffer has a quiescent current of 140

µA while roughly

100

µA is drawn by the microphone itself. A CR2032 3 V

lithium battery has a life expectancy of 160 mA hours, which

means this circuit could run continuously for 640 hours on a

single battery.

SPICE Macro-Model

* OP181 SPICE Macro-model

* 9/96, Ver. 1

*

* Copyright 1996 by Analog Devices

*

* Refer to “README.DOC” file for License Statement. Use of this

* model indicates your acceptance of the terms and provisions in

* the License Statement.

*

* Node Assignments

*

noninverting input

*

|

inverting input

*

|

|

positive supply

*

|

|

|

negative supply

*

|

|

|

|

output

*|

|

|

|

|

*|

|

|

|

|

.SUBCKT OP181

1

2

99

50

45

*

* INPUT STAGE

*

Q1

41

3PIX

Q2

67

5PIX

I1

99

8

1.28E-6

EOS

7

2

POLY(1)

(12, 98) 80E-6 1

IOS

1

2

1E-10

RC1

4

50

500E3

RC2

6

50

500E3

RE1

3

8

108

RE2

5

8

108

V1

99

13

DC .9

V2

99

14

DC .9

D1

3

13

DX

D2

5

14

DX

*

* CMRR 76dB,

ZERO AT 1kHz

*